Here's two more counterarguments.

- With capped content, you'd be forced to store low level equipment for each job you intend to enter the dungeon with. Given the limited storage space we have, that's a definite no-go; and the scaling algorithms currently in place do not fully scale you high level gear to be exactly equal to low-level gear worn at the appropriate level. Counterargument: None so far
The game seems to have already been designed with level caps in mind. The vast majority of gear already scales down in level as do all abilities. So even if you enter as a scaled down Lv50 with high rank gear, your additional high level abilities will help balance you compared to someone of correct level with gear appropriate to their level. You will never need to hold on to "capped gear-sets."

- People would have to re-do all their macros. Which is not just a hassle, but impossible for players with several jobs at max level, because there simply is no macro space left to accomodate all the gearswaps, ability settings and different battle-macros. Counter argument: None so far
As stated above, most gear and ALL abilities can be used at any level. There's no reason to ever edit macros. A Lv50 can enter with their exact skill-set and gear-set as they normally use and play their class exactly as they would outside of a level-capped instance, just scaled down.

Both arguments seem very similar.